The present invention relates to external fixation devices, and in particular, to distractor mechanisms and hinges for external fixation devices.
External fixation of bone fractures is well known in the art. Many different external fixation devices have been developed, virtually all of which in one form or another use multiple transverse fixation wires or half-pins which extend through, or are embedded in, respectively, the bone and soft tissue surrounding the bone, and connect to various types of supporting elements, such as rings, half-rings, arches or bars.
Aside from providing stable fixation of bone fragments to promote proper fracture healing, external fixation devices also provide means for transporting the bone fragments in order to correct length discrepancies or angular deformities of the bone. One of the more common external fixation devices used for such purposes, often referred to as the Ilizarov External Fixator, includes three basic elements: multiple rings (or arches) disposed coaxially about the bone segments to be fixated; transverse wires or half-pins for fixating the bone segments to the rings (or arches); and distractor mechanisms. For angular distraction, the external fixator further includes pivots, or hinges, to define an axis of rotation perpendicular to the plane of the bone deformity to allow rotation of the bone segment.
Hence, for linear distraction, a typical basic assembly includes proximal and distal rings, or ring sets, connected by three or four distractor mechanisms; and for angular distraction, a typical basic assembly includes proximal and distal rings, or ring sets, connected by two hinges and one distractor mechanism.
A conventional distractor mechanism for linear distraction consists of a threaded rod which is fastened to each of the rings by the use of double nut assemblies, i.e. nuts which are threaded onto the rod and tightened against either side of each ring. A conventional distractor mechanism for angular distraction consists of two hinge segments which are fastened to the proximal and distal rings, and are coupled to each other via a threaded distraction rod. A conventional basic hinge assembly used for angular distraction consists of two hinge segments which are fastened directly to the rings and connected to each other with a nut and bolt.
The current conventional external fixator elements (distractors and hinges) present a number of problems. First, because the connection points for the distractor mechanisms and hinge assemblies, i.e. the holes in the rings and hinge members, must be slightly larger than the diameter of the threaded component (rods and bolts), the resulting assembly is always unstable. In other words, during installation, adjustment or distraction, a certain amount of mechanical "free play" will be present. During the treatment period of the patient, this can and often does cause pain.
Currently, there are two methods used in seeking to compensate for this problem. One method is to loosen and retighten all hinge and distractor nuts before and after each angular correction or linear distraction maneuver. This can be effective, but is time consuming and frustrating for the patient. Plus, the torque which must be applied to loosen and retighten the hardware can adversely affect the healing process and cause pain for the patient. The second method involves the use of nylock nuts in the hinges and distractor mechanisms to achieve some degree of radial stability by imposing an axial preload on the rotating elements. The mechanical "free play" is still there, but is damped by frictional forces from the fasteners. However, even when the hinge is precisely adjusted, the potential for an unstable frame is still present due to the over-sized hole diameters.
A second problem arises during each distraction maneuver, particularly during an angular distraction. Each incremental distraction takes a great deal of time, and the procedure is generally too complex for young patients to perform reliably. With so many components, i.e. plates, hinge segments, rods, bolts and nuts, to loosen, turn and retighten, it is difficult for the patient to remember the adjustment sequence and maintain a consistent distraction rate. Moreover, if any of the nuts are accidentally left untightened, instability, and therefore, pain and delayed healing, can result.
Accordingly, it would be desirable to have an external fixation device with a distraction mechanism which allows adjustable distraction while simultaneously providing a rigid structure, i.e. both radial and axial stability of the overall assembly.